Pressure sensitive adhesive sheet, method of protecting semiconductor wafer surface and method of processing work

ABSTRACT

Provided is a pressure sensitive adhesive sheet that does not adhere to other apparatuses, even when it is used in a manufacturing scheme using heat treatment or treatment involving heat generation. More particularly, the pressure sensitive adhesive sheet is suited for semiconductor wafer processing, possessing unprecedented high-temperature heat resistance. The pressure sensitive adhesive sheet can be used as a surface protective sheet, a dicing sheet or a pickup sheet, by imparting properties such as a protective function of an uneven circuit surface or expanding properties. The pressure sensitive adhesive sheet comprises a base material obtained by film-forming and curing a first curable resin, a top coat layer formed on the base material by coating and curing a second curable resin, and a pressure sensitive adhesive layer formed on the opposite side of the base material.

TECHNICAL FIELD

The present invention relates generally to pressure sensitive adhesivesheets, and more particularly to pressure sensitive adhesive sheets thatcan be used as surface protective sheets, dicing sheets or pickup sheetsand suitably applied to manufacturing processes including heat treatmentor treatment involving heat generation after attachment to semiconductorwafers.

BACKGROUND ART

In a process for manufacturing semiconductor devices, pressure sensitiveadhesive sheets such as surface protective sheets or dicing sheets areused in a backside grinding process or a dicing process. Heat resistancehas been not required for the pressure sensitive adhesive sheets for usein these applications, because when pressure sensitive adhesive sheetsare applied to semiconductor wafers, washing water has been used toprevent an increase in the temperature during the processing.

Recently, it has been proposed to form circuitry on both sides ofsemiconductor wafers to enhance the integration density. In this case,after finishing backside grinding, wafers are subjected to processingsuch as circuitry formation on the ground side of wafers, while thewafers are being held on surface protecting sheets. Various treatmentsthat involve heating such as etching are performed for the circuitryformation. Consequently, surface protective sheets are also exposed tothe heat.

However, conventional surface protective sheets using soft basematerials occur melting or softening of the base materials by heatingand adhesion of the base materials to the holding part of an etchingapparatus.

Therefore, surface protective sheets in which the base materials do notadhere to other apparatuses by heating are required.

In order to eliminate such problems as adhesion by heating, the use ofrigid films with a high melting point such as polyethylene terephthalateor polyethylene naphthalate has been proposed. However, these rigidfilms are inferior in protective performance of the circuit surface.Moreover, even these high-melting-point films sometimes have sufferedfrom shrinkage by heating to cause warpage of wafers after processing.

Moreover, after the backside grinding of semiconductor wafers, it hasbeen proposed to form films having various functions such as insulatingfilms or anisotropic conductive adhesive layers on the backside ofwafers or on the circuit surface. Films having these functions areformed on the wafers by thermo-compression bonding of functional filmscomprised of thermo-adhesive films. At this time, when the wafer is thinand thus easily broken by pressure, the formation of the films may beperformed in a configuration where the opposite side of the wafer isapplied with a pressure sensitive adhesive sheet such as a surfaceprotective sheet or a dicing sheet to reinforce the wafer.

However, the pressure sensitive adhesive sheets to be used forprocessing semiconductor wafers at a high temperature are hardly knownto date. After diligent study, the present inventors have found that theheat resistance is improved by using highly crosslinked films instead ofthermoplastic films such as polyolefin or the like as the base materialsfor use in the pressure sensitive adhesive sheets. The structure of eachof these pressure sensitive adhesive sheets is disclosed by the presentapplicant, for example, in Japanese Patent Laid-Open No. 9-253964,Japanese Patent Laid-Open No. 10-337823, and Japanese Patent Laid-OpenNo. 2002-141306.

However, even for these pressure sensitive adhesive sheets., there hasremained a problem of heat-adhesion to a heated table (wafer-holdingpart), when the base materials are adjusted to be soft in order toimpart protective properties for circuit surfaces or expandingproperties.

The present invention has been made in view of the above described priorarts, and an object of the present invention is to provide a pressuresensitive adhesive sheet that does not adhere to other apparatuses andthe like, even when it is applied to a manufacturing including heattreatment or treatment involving heat generation. It is a further objectof the present invention to provide a pressure sensitive adhesive sheetfor semiconductor wafer processing or the like having unprecedentedhigh-temperature resistance, which can be used as a surface protectivesheet, a dicing sheet or a pickup sheet, by imparting properties such asprotective function of a circuit surface or expanding properties.

DISCLOSURE OF INVENTION

A pressure sensitive adhesive sheet according to the present inventionis characterized by comprising a base material obtained by film-formingand curing a first curable resin, a top coat layer formed on the basematerial by coating and curing a second curable resin, and a pressuresensitive adhesive layer formed on the opposite side of the basematerial.

In the pressure sensitive adhesive sheet described above, a cured resinforming the top coat layer preferably has no peak of 0.1 J/g or more inthe DSC measurement from 50 to 200° C.

In addition, the above described base material preferably has a Young'smodulus of 50 to 5,000 MPa.

A method for protecting the surface of a semiconductor wafer accordingto the present invention is characterized by comprising the steps ofapplying the above described pressure sensitive adhesive sheet to thecircuit surface of a semiconductor wafer having circuitry formed on thefront side, and grinding the backside of the semiconductor wafer.

In the method for protecting the surface of a semiconductor waferaccording to the present invention, a semiconductor wafer applied with apressure sensitive adhesive sheet can be subjected to heat treatment ortreatment involving heat generation before or after grinding thesemiconductor wafer.

Here, the heat treatment may include, for example, thermo-compressionbonding of a thermo-adhesive film on the ground side of a semiconductorwafer. Moreover, the treatment involving heat generation may include,for example, the treatment selected from vacuum deposition, sputteringand plasma etching applied to the ground side of a semiconductor wafer.

A method for processing a workpiece according to the present inventionis characterized by comprising the steps of fixing a workpiece with theabove described pressure sensitive adhesive sheet and picking up theworkpiece.

Moreover, a workpiece applied with a pressure sensitive adhesive sheetmay be subjected to heat treatment or treatment involving heatgeneration, before the picking up of the workpiece.

In particular, the present invention realizes a method for processing aworkpiece wherein a workpiece fixed to a pressure sensitive adhesivesheet is applied with a thermo-adhesive film by thermo-compressionbonding; the workpiece is diced together with the thermo-adhesive film;and then the diced workpiece is picked up and thermally adhered to asubstrate via the thermo-adhesive film.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in further detail below.

The pressure sensitive adhesive sheet according to the present inventionis composed of a base material, a top coat layer formed thereon and apressure sensitive adhesive layer formed on the opposite side thereof.The base material, the top coat layer and the pressure sensitiveadhesive layer will be described in detail below.

(Base Material)

The base material is made of a film obtained by film-forming and curinga first curable resin to be described below. The film desirably has aYoung's modulus of from 50 to 5,000 MPa, preferably from 60 to 4,000MPa, most preferably from 80 to 3,000 MPa. Moreover, the thickness ofthe base material is preferably from 1 to 1,000 μm, more preferably from10 to 800 μm, most preferably from about 20 to 500 μm but not restrictedthereto.

Energy ray-curable resins, thermosetting resins or the like are used asa first curable resin to be used as a raw material for the basematerial, and energy ray-curable resins are preferably used. If the basematerial is prepared by curing a curable resin, the base material willbecome less susceptible to temperature-induced deformation such asmelting by heating, and its heat resistance is improved.

For example, a resin composition mainly composed of an energy-raypolymerizable urethane acrylate oligomer is suitably used as an energyray-curable resin. The molecular weight of the urethane acrylateoligomer suitably used in the present invention ranges from 1,000 to50,000, more preferably from 2,000 to 30,000. The urethane acrylateoligomer can be used singly or in combination of two or more.

It is often difficult to form a film only by the above describedurethane acrylate oligomer. Therefore, the urethane acrylate oligomer isgenerally diluted with an energy ray-polymerizable monomer and processedinto film-form, thereby cured to obtain a film. The energyray-polymerizable monomer has an energy ray-polymerizable double bond inits molecule, and particularly acrylate compounds having a relativelybulky group such as isobornyl (meth)acrylate, dicyclopentenyl(meth)acrylate, and phenyl hydroxypropyl acrylate are suitably used inthe present invention.

The above described energy ray-polymerizable monomer is used in a ratioof preferably from 5 to 900 parts by weight, more preferably from 10 to500 parts by weight, most preferably from 30 to 200 parts by weight, per100 parts by weight of the urethane acrylate oligomer.

When forming the base material from the above described energyray-curable resin, the time of polymerization and curing by energy-rayirradiation, and the amount of irradiation can be reduced byincorporating a photopolymerization initiator into the resin.

The photopolymerization initiator is used in an amount of preferablyfrom 0.05 to 15 parts by weight, more preferably from 0.1 to 10 parts byweight, most preferably from 0.5 to 5 parts by weight, per 100 parts byweight of the total resin.

The above described curable resin can be selected from variouscombinations of oligomers and monomers formulated so as to have abovedescribed Young's modulus.

Moreover, the above described resin may contain additives includinginorganic fillers such as calcium carbonate, silica and mica; metallicfillers such as iron and lead; colorants such as pigments and dye; andthe like.

A method of forming the base material comprises casting a first curableresin in liquid-form on a process film into thin film-form, convertingit into a film by predetermined means and removing the process film.This method reduces the stress applied to a resin during film-formingand minimizes dimensional changes with the lapse of time or heating..Moreover, as solid impurities are easily removed, formation of fish eyesin the formed film is reduced. Thereby, the uniformity of film thicknessis improved, resulting in a thickness accuracy of generally within 2%.

Moreover, both sides of the base material, that is, the surface on whicha top coat layer is formed and another surface on which a pressuresensitive adhesive layer is formed, may be subjected to corona treatmentor primer treatment to form another layer to thereby enhance bondingstrength to these layers.

The film formed from the raw materials and by the method as describedabove may exhibit properties excellent in stress relaxation properties.When a film excellent in stress relaxation properties is used as thebase material for a pressure sensitive adhesive sheet, the residualstress to be generated when applied to an adherend can be quicklyeliminated and does not adversely affect the subsequent processing.Therefore, when the pressure sensitive adhesive sheet is used forprotecting a semiconductor wafer that is subjected to grinding to anextra-thin thickness, the semiconductor wafer does not exhibit warpagebecause of the stress relaxation.

The stress relaxation properties of the base material are specificallyrepresented by the percentage of stress relaxation after one minute at10% elongation in a tensile test, and it is preferably 40% or more, morepreferably 50% or more, most preferably 60% or more. The higherpercentage of stress relaxation of the base material is more preferable,and its upper limit, which is theoretically 100%, may be 99.9%, 99% or95%.

(Top Coat Layer)

A top coat layer is coated on one side of a base material and furtherimproves the heat resistance of the base material. The top coat layer isobtained by film-forming and curing of a second curable resin.

The cured top coat layer preferably has no peak of 0.1 J/g or more inthe DSC (differential scanning calorimetry) measurement-from 50 to 200°C. If there is a peak in the DSC measurement (generation or absorptionof heat occurs), the top coat layer may be easily deformed by heating toincrease the contact area, resulting in adhesion to a heater plate orthe like.

Moreover, a rough surface of the top coat layer is preferred, because itcan reduce the contact area with a heater plate and further prevent theadhesion during heating. The top coat layer has a surface roughness Rzof preferably from 0.05 to 1.0 μm, more preferably from 0.1 to 0.5 μm.If the top coat layer is too rough, it may be slippery and may impairworkability in some steps of wafer processing.

Energy ray-curable resins, thermosetting resins or the like are used asa second curable resin for forming such a top coat layer, similar to thefirst curable resin used for forming the base material, and energyray-curable resins are preferably used. It is preferable to select thesecond curable resin such that it has a higher crosslinking densityafter curing than that of the first curable resin. This minimizes theoccurrence of generation or absorption of heat in the DSC measurement.

When the energy ray-curable resin is used as the second curable resin,it is preferable to increase the content of a multifunctional energyray-curable compound having low molecular weight. Such energyray-curable compounds to be used include, for example,trimethylolpropane triacrylate, pentaerythritol triacrylate,pentaerythritol tetraacrylate, dipentaerythritol monohydroxypentaacrylate, and dipentaerythritol hexaacrylate.

When the second curable resin is composed of the multifunctional energyray-curable compound having low molecular weight, the adhesion to thebase material may decrease. For improving the adhesion to the basematerial, a binder component may be added in the second curable resin.The use of the binder component may cause generation or absorption ofheat in the DSC measurement caused by a polymer in the binder. However,if the binder component and the energy ray-curable compound aresufficiently compatible, the micro-molecular motion of the bindercomponent after curing can be suppressed, and the amount of heatgeneration or heat absorption can be reduced, to thereby prevent theadhesion to a heater plate or the like. Examples of such bindercomponents include acrylic resins, polyester resins, urethane resins andpolyamide resins.

Moreover, the second curable resins may be polymers having an energy raycurable-functional group on the side chain. The use of these polymers assecond curable resins can improve the adhesion to base materials withoutreducing crosslinking density. The polymers that can be used mayinclude, for example, those having a main chain of acrylic polymer and aside chain of an energy ray-curable double bond or an epoxy group as afunctional group.

The surface roughness of the top coat layer can be appropriatelyadjusted by the amount of fillers to be added. The fillers to be usedinclude, for example, inorganic fillers such as calcium carbonate,silica and mica; and metallic fillers such as iron and lead. The surfaceroughness is increased by increasing the amount of the fillers. Theamount of the fillers to be added is different depending on the type ofthe fillers, and suitable amount is generally from 0 to 200 parts byweight, more preferably from about 5 to 100 parts by weight, per 100parts by weight of the curable resin.

The top coat layer can be formed by directly film-forming and curing thesecond curable resin on the above described base material. In addition,a base material with a top coat layer can be formed by casting thesecond curable resin in liquid-form on a casting film and furthercasting a first curable resin thereon. The curing of the top coat andthe base material may be performed immediately after each film-formingor may be performed together after the film-forming of the basematerial. When the top coat is formed by casting, the surface roughnessdepends on the roughness of a casting film, so it is preferable toselect a casting film having appropriate roughness.

The thickness of the top coat layer is not particularly limited, and ispreferably from 0.2 to 20 μm, most preferably from about 0.5 to 5 μm.

(Pressure Sensitive Adhesive Layer)

In the pressure sensitive adhesive sheet of the present invention, apressure sensitive adhesive layer is formed on the surface of a basematerial opposite to the surface on which a top coat layer is formed.

The pressure sensitive adhesive layer may be formed from strong pressuresensitive adhesives for general purpose use, energy ray-curable pressuresensitive adhesives frequently used for wafer processing, or removablepressure sensitive adhesives for general purpose use. It is preferableto form the pressure sensitive adhesive layer by the energy raycurable-pressure sensitive adhesives particularly in the presentinvention.

Generally, such energy ray-curable pressure sensitive adhesives aremainly composed of acrylic pressure sensitive adhesives and energyray-curable compounds.

Low molecular weight compounds having two or more energyray-polymerizable carbon-carbon double bonds in its molecule which canform three-dimensional network by photoirradiation as disclosed forexample in Japanese Patent Laid-Open No. 60-196956 and Japanese PatentLaid-Open No. 60-223139 are widely used as the energy ray-curablecompounds for the energy ray-curable pressure sensitive adhesives.Specifically, trimethylolpropane triacrylate, pentaerythritoltriacrylate, pentaerythritol tetraacrylate, dipentaerythritolmonohydroxy pentaacrylate, dipentaerythritol hexaarylate,1,4-butyleneglycol diacrylate, 1,6-hexanediol diacrylate,polyethyleneglycol diacrylate, or oligomers such as oligoester acrylatesor urethane acrylates are used.

The compounding ratio of the energy ray-polymerizable compound to theacrylic pressure sensitive adhesive in the energy ray-curable pressuresensitive adhesive is preferably from 50 to 200 parts-by weight of theenergy ray-polymerizable compound per 100 parts by weight of the acrylicpressure sensitive adhesive. When they are used in these ranges, theresultant pressure sensitive adhesive sheet has large initial adhesivestrength, and the adhesive strength sharply decreases after energy-rayirradiation. Therefore, the pressure sensitive adhesive sheet can beeasily removed from an adherend by energy-ray irradiation.

Moreover, energy ray-curable pressure sensitive adhesive layers may beformed from energy ray-curable copolymers having energyray-polymerizable groups on the side chain. These energy ray-curablecopolymers have both tackiness and energy ray-curability. The details ofthe energy ray-curable copolymers having energy ray-polymerizable groupson the side chain are disclosed, for example, in Japanese PatentLaid-Open No. 5-32946 and Japanese Patent Laid-Open No. 8-27239.

The pressure sensitive adhesive layer has a thickness, which variesdepending on materials, of generally from about 3 to 100 μm, preferablyfrom about 10 to 50 μm.

The pressure sensitive adhesive sheet of the present invention can beobtained using generally known coaters such as roll coaters, knifecoaters, roll-knife coaters, reverse coaters and die coaters, whereinthe pressure sensitive adhesive layer is formed by coating and dryingthe above-described pressure sensitive adhesives so as to have asuitable thickness on the base material opposite to the top coat layer,or the pressure sensitive adhesive layer is formed on a release sheetand transferred to the surface of the base material.

Next, methods for processing workpieces according to the presentinvention will be described, taking a method for protecting surfaces., adicing method and a pickup method, particularly for processingsemiconductor wafers, as examples. However, in the present invention,the workpieces to be processed are not limited to semiconductor wafers.For example, the present invention can be applied to the processing ofvarious ceramics, glass, metal or the like.

(Method for Protecting Surfaces)

The pressure sensitive adhesive sheet according to the present inventionhas a specific top coat layer formed on one surface of the basematerial, as described above. This top coat layer has high heatresistance. Therefore, even in the case where the pressure sensitiveadhesive sheet is applied to a process including the treatment involvingheat treatment, if the top coat layer is placed in contact with themember that has high temperature in an apparatus or the like (a sampletable in etching or the bottom of an oven), the pressure sensitiveadhesive sheet will not adhere to these apparatuses.

Such pressure sensitive adhesive sheets can be used even in themanufacturing of semiconductor wafers including heat treatment ortreatment involving heat generation in which the application of pressuresensitive adhesive sheets has been conventionally impossible.

More specifically, the pressure sensitive adhesive sheets can besuitably applied to manufacturing processes such as (1) and (2) asdescribed below as surface protective sheets for semiconductor wafers.

(1) Surface Protective Sheets in the Treatment Involving Heat GenerationSuch as Plasma Etching to the Ground Surface of Semiconductor Wafers

In some cases, semiconductor wafers are subjected to plasma etchingafter backside grinding in order to remove a debris layer which maycause package crack. For performing plasma etching, semiconductor waferswith pressure sensitive adhesive sheets for protecting surfaces thereonare sent to-a plasma etching apparatus so as to prevent breakage duringtransportation, because semiconductor wafers are often ground toextremely thin thickness. The semiconductor wafers generate heat toapproximately 180° C. at the etching, resulting in heating pressuresensitive adhesive sheets too. If conventional pressure sensitiveadhesive sheets are used as surface protective sheets, base materialsmay melt or soften to cause the pressure sensitive adhesive sheets to beadhered to a sample table in the etching apparatus. However, when thepressure sensitive adhesive sheet of the present invention is used as asurface protective sheet, neither a base material nor a top coat layermelts, and the top coat layer is hard to be softened, so that thepressure sensitive adhesive sheet will not cause fusion or adhesion onthe sample table of the etching apparatus.

As a typical manufacturing, the pressure sensitive adhesive sheet forprotecting surfaces is applied on the circuit surface of a semiconductorwafer on which circuitry is formed, and the semiconductor wafer isground to a predetermined thickness by a grinding apparatus. Thesemiconductor wafer is transferred to the plasma etching apparatus toremove the debris layer formed on the ground surface by etching, withoutremoving the pressure sensitive adhesive sheet. Thereafter, thesemiconductor wafer is taken out of the plasma etching apparatus, dicedby a predetermined process, and packaged to manufacture a semiconductordevice.

The treatment involving heat generation other than the plasma etchingincludes processes for forming films by physical deposition such asvacuum deposition or sputtering, or by CVD process.

(2) Surface Protective Sheets in the Lamination of Thermo-Adhesive ResinFilms

In some cases, resin films are provided on the backside of wafers, inorder to impart sufficient strength to wafers (finally chips), or toimprove adhesion with mold resins, or to be used for bonding to leadframes. The resin film may be formed on the wafer by a spin coater, ormay be laminated to the wafer by applying a film having thermo-adhesiveproperties to the wafer by thermo-compression bonding using a heaterroller or the like. Since for example a thermoplastic polyimide havingheat resistance is typically used as the thermo-adhesive resin film,heating to approximately 180° C. is adopted as the condition of thethermo-compression bonding. During the thermo-compression bonding, thecircuit surface in contact with the surface of a fixed table isprotected by applying the pressure sensitive adhesive sheet forprotecting surfaces. If a conventional pressure sensitive adhesive sheetis used as a surface protective sheet, a base material may melt orsoften to cause adhesion of the pressure sensitive adhesive sheet to afixed table. However, when the pressure sensitive adhesive sheet of thepresent invention is used as a surface protective sheet, neither a basematerial nor a top coat layer melts, and the top coat layer is hard tobe softened, so that the pressure sensitive adhesive sheet will notcause fusion or adhesion on the fixed table.

As a typical manufacturing, the pressure sensitive adhesive sheet forprotecting surfaces is applied on the circuit surface of a semiconductorwafer on which circuitry is formed, and the semiconductor wafer isground to a predetermined thickness by a grinding apparatus. Thesemiconductor wafer is transferred to a laminator capable of heattreatment without removing the pressure sensitive adhesive sheet toundergo thermo-compression bonding of a desired thermo-adhesive resinfilm on a desired portion of the semiconductor wafer. Then, thesemiconductor wafer is taken out of the laminator, diced-in apredetermined process, and packaged to manufacture a semiconductordevice.

(Dicing Method, Pickup Method)

Moreover, the pressure sensitive adhesive sheet of the present inventioncan be suitably applied to manufacturing processes such as (3) and (4)as a dicing sheet or a pickup sheet as described below.

(3) Dicing Sheets in a Process to Form Anisotropic Conductive AdhesiveFilms

When semiconductor chips are mounted on chip substrates by a flip-chipbond system, there is a method securing the conduction between the chipsand the substrates by anisotropic conductive adhesive films. In such acase, the anisotropic conductive film may be formed on the substrateside, but it is preferable to provide it on the wafer side because theanisotropic conductive adhesive film can be cut into a chip size at thesame time as the chip-dicing in a dicing process.

When the thickness of the wafer is thin, a dicing sheet is applied onthe ground surface after grinding the backside of the wafer, and then aprotective sheet is removed from the circuit surface of the wafer, so asto prevent the breakage of the wafer. The anisotropic conductiveadhesive film will be formed on the circuit surface of the wafer whilethe dicing sheet is applied on the ground surface. When the anisotropicconductive adhesive film is formed on the circuit surface of the wafer,the film is thermally bonded to follow the irregularity of the circuitsurface to thereby bonding thereon, so as to prevent formation of theremaining air (void) at the interface between the film and theirregularity of the circuit surface. The dicing sheet side is broughtinto contact with a heating table at the thermo-compression bonding.Therefore, as for a conventional dicing sheet, its base material maymelt or soften to cause adhesion of the pressure sensitive adhesivesheet to a heating table.

However, when the pressure sensitive adhesive sheet of the presentinvention is used as a dicing sheet, neither a base material nor a topcoat layer melts, and the top coat layer is hard to be softened, so thatthe pressure sensitive adhesive sheet-will not cause fusion or adhesionon the heating table.

As a typical manufacturing, the pressure sensitive adhesive sheet forprotecting surfaces is applied on the circuit surface of a semiconductorwafer and its backside is ground to a predetermined thickness, and thenthe pressure sensitive adhesive sheet for dicing is applied on theground surface. The pressure sensitive adhesive sheet for protectingsurfaces is removed from the circuit surface of the wafer, and thesemiconductor wafer in this state is transferred to a laminator capableof heat treatment to undergo thermo-compression bonding of theanisotropic conductive adhesive film on the circuit surface of thewafer. Then, the semiconductor wafer is taken out of the laminator,diced together with the anisotropic conductive adhesive film to beconverted to chips. The chips are subjected to flip-chip bonding tosubstrates via the anisotropic conductive adhesive film and thenpackaged to manufacture a semiconductor device.

By the way, the present invention is not limited to the lamination ofthe anisotropic conductive adhesive film, but can be applied to theprocess for providing an insulating thermo-adhesive film. In this case,the thermo-adhesive film is allowed to flow at flip-chip bonding tobring the chip into contact with the electrode of a substrate forconduction.

(4) Dicing Sheets in a Transfer Process of Wafers (Chips) UsingHeat-Releasable Sheets (Pickup Sheets)

When semiconductor wafers are ground to an extreme thinness with fixingthe wafers to hard plates such as glass, the accuracy of thickness canbe improved, and the breakage can be prevented. In such a case,typically, the wafers are fixed on the hard plates by double-sidedpressure sensitive adhesive sheets. In order to facilitate the removalof the wafers from the hard plates after finishing the grinding,heat-deformable pressure sensitive adhesive sheets are used as thedouble-sided pressure sensitive adhesive sheets. Various heat-deformablepressure sensitive adhesive sheets have been devised. As an example,heat-shrinkable properties are utilized to deform a pressure sensitiveadhesive sheet to reduce the contact area with a wafer to facilitate theremoval of the wafer (e.g., Adwill N series made by Lintec Corporation).As another example, there is a double-sided pressure sensitive adhesivesheet using a heat-expandable pressure sensitive adhesive as a pressuresensitive adhesive layer (e.g., REVALPHA made by Nitto DenkoCorporation), in which the pressure sensitive adhesive layer is expandedby heating to reduce the contact area with a wafer to facilitate theremoval of the wafer.

When the wafer is directly removed from a hard plate, the possibility ofbreakage of a wafer is large. Accordingly, there is devised a method forpreventing the breakage by applying a dicing sheet to be used in thenext process, before the wafer is removed from the hard plate (JapanesePatent Laid-Open No. 2001-217212).

The heating for removing the wafer is conducted in an oven or the like,and at this time the dicing sheet is brought into contact with thebottom of the oven. As for the dicing sheet using conventional soft basematerials, this may cause the base material to be melted or softened bythe heating to cause the base materials to be adhered to the bottom ofthe oven.

However, when the pressure sensitive adhesive sheet of the presentinvention is used as a dicing sheet, neither a base material nor a topcoat layer melts, and the top coat layer is hard to be softened, so thatthe pressure sensitive adhesive sheet will not cause fusion or adhesionon the bottom of the oven.

As a typical manufacturing, the semiconductor wafer is applied and fixedto a hard plate such as a glass plate using a heat-deformabledouble-sided pressure sensitive adhesive sheet. The wafer is subjectedto backside grinding while fixed to the hard plate, and then a dicingsheet comprised of the pressure sensitive adhesive sheet of the presentinvention is applied to the wafer surface. When the wafer in this stateis introduced into an oven for heating, the double-sided pressuresensitive adhesive sheet is deformed to remove the wafer from thedouble-sided pressure sensitive adhesive sheet, resulting in a statethat the wafer is adhered only with the dicing sheet. This wafer istaken out of the oven, diced and then subjected to a predeterminedpackaging process to manufacture a semiconductor device.

In the above method, a manufacturing in which only a backside-grindingprocess is conducted on a hard plate has been described. However, adicing process may be performed on the hard plate, following thebackside-grinding process. In this case, the pressure sensitive adhesivesheet of the present invention may be, instead of a dicing sheet, apressure sensitive adhesive sheet dedicated to the pickup process(pickup sheet). The pickup sheet refers to a pressure sensitive adhesivesheet that is applied to a wafer constructed in chip form, and istailored to the function for picking up chips.

INDUSTRIAL APPLICABILITY

The present invention provides a pressure sensitive adhesive sheet thatwill not adhere to other apparatuses and the like even when applied to amanufacturing including heat treatment or treatment involving heatgeneration. The present invention further provides a pressure sensitiveadhesive sheet for semiconductor wafer processing having unprecedentedhigh-temperature heat resistance, which can be used as a surfaceprotective sheet, a dicing sheet or a pickup sheet, by impartingproperties such as protective function or expanding properties forcircuit surfaces.

EXAMPLES

The present invention will be described below with reference toExamples, but the present invention is not limited to these Examples. Inthe Examples and Comparative Examples below, the evaluation of “Young'smodulus”, “Surface roughness”, “Stress relaxation ratio”, “DSC peakamount”, “High temperature adhesion” and “Wafer warpage” was made asfollows:

“Young's Modulus”

The base materials of pressure sensitive adhesive sheets prepared inExamples or Comparative Examples were measured in accordance with JISK-7127 at a test speed of 200 mm/minute.

“Surface Roughness”

The surfaces of top coat layers of pressure sensitive adhesive sheetsprepared in Examples or Comparative Examples were measured for theaverage roughness (Rz) in accordance with JIS B-0601. Note that inComparative Examples 1 and 3, the surfaces of base materials weremeasured.

“Stress Relaxation Ratio”

Each laminate of a base material and a top coat layer prepared inExamples or Comparative Examples is cut to a width of 15 mm and a lengthof 100 mm to obtain a specimen. This specimen is elongated usingTENSILON RTA-100 made by Orientec Corporation at a speed of 200mm/minute, and the stress relaxation ratio is calculated from stress Aat an elongation of 10% and stress B at one minute after the elongationaccording to the expression: (A−B)/A×100 (%).

“DSC Peak Amount”

The same formulations as the coating agents for the top coat layers ofthe pressure sensitive adhesive sheets prepared in Examples orComparative Examples were dried and UV-cured to prepare samples for DSCmeasurement. A sample of about 10 mg was served for measurement, inwhich a differential scanning calorimeter (Pyris I made by Perkin Elmer,Inc.) was used for the measurement from room temperature to 220° C. at atemperature increasing rate of 10° C./minute. The amount of energy ofmaximum heat absorption or heat generation occurring in the range of 50to 200° C. was defined as the DSC peak amount.

“High Temperature Adhesion”

A pressure sensitive adhesive sheet (50 mm×50 mm) of Examples orComparative Examples was placed on a mirror-polished stainless steelplate with the top coat side facing the plate. The stainless steel platewith the pressure sensitive adhesive sheet was set and heated on a hotplate of 180° C. for five minutes while applying a load of 100 g. Thestainless steel plate was cooled at room temperature for one hour, andthen turned upside down. When a pressure sensitive adhesive sheet freelyfell, it was defined to be in a state of “not adhered,” and when thepressure sensitive adhesive sheet could not freely fall, it was definedto be in a state of “adhered.” Note that in Comparative Examples 1 and 3the pressure sensitive adhesive sheet was placed on the stainless steelplate with the base material side facing the plate.

“Wafer Warpage”

A pressure sensitive adhesive sheet prepared in Examples and ComparativeExamples was applied to a Silicon wafer having a diameter of 200 mm anda thickness of 725 μm, and the wafer was subjected to grinding to 100μm. Subsequently, the wafer was set and heated on a hot plate of 180° C.for five minutes with the pressure sensitive adhesive sheet side facingthe hot plate. The wafer was cooled at room temperature for one hourwithout removing the pressure sensitive adhesive sheet, and placed on awafer holding plate with the pressure sensitive adhesive sheet surfacefacing upward, leaving the wafer at rest. The height of the wafer wasmeasured at 17 measurement points, defining the top surface of the waferholding plate as the zero point, and the difference of the maximum andthe minimum of the measurement values was defined as the amount ofwarpage.

Example 1

1-1) Following formulations were used as an ultraviolet ray-curablecoating agent for forming a top coat layer.

urethane acrylate oligomer (molecular weight (Mw) about 1,000): 50 partsby weight

dipentaerythritol hexaacrylate: 50 parts by weight

photoinitiator (IRGACURE 184 made by Ciba Specialty Chemicals Inc.): 4.0parts by weight

silica filler (SNOWTEX-UP made by Nissan Chemical Industries Ltd.): 30parts by weight

The above coating agent was coated using a Meyer bar on a casting film(SP-PET38E made by Lintec Corporation) made of a polyethyleneterephthalate film (hereinafter referred to as a PET film) having athickness of 38 μm being subjected release treatment with a siliconeresin, cured by ultraviolet ray irradiation (250 mJ/cm²) to form acoating film of only a top coat layer having a thickness of 2 μm on thecasting film.

1-2) Subsequently, following formulations were used as a coating agentfor forming a base material.

urethane acrylate oligomer (molecular weight (Mw) about 5,000): 50 partsby weight

isobornyl acrylate: 50 parts by weight

photoinitiator (IRGACURE 184): 2.0 parts by weight

This coating agent was coated using a fountain die coater on the topcoat layer on a casting film prepared in 1-1), cured by ultraviolet rayirradiation (250 mJ/cm²) to form a base material-made of a cured coatingfilm having a thickness of 157 μm on the top coat layer.

Next, an ultraviolet ray-curable pressure sensitive adhesive offollowing formulations was used as a coating agent for forming pressuresensitive adhesive layer.

an adduct product of 100 parts by weight of a copolymer (weight averagemolecular weight of about 500,000) consisting of 62 parts by weight ofn-butyl acrylate, 10 parts by weight of methyl methacrylate and 28 partsby weight of hydroxyethyl acrylate, with 30 parts by weight ofmethacryloyloxyethyl isocyanate

0.3 parts by weight of photopolymerization initiator comprised ofbis(2,4,6-trimethylbenzoyl)phenylphosphine oxide

0.3 parts by weight of a crosslinking agent comprised of an adductproduct of tolylenediisocyanate with trimethylolpropane

This coating agent was coated on a release film (SP-PET3811(S))manufactured by Lintec Corporation) made of a PET film having athickness of 38 μm being subjected to release treatment with siliconeresin, dried and transferred to the surface of the base materialprepared in 1-2) on the side without the top coat, to thereby preparinga pressure sensitive adhesive sheet having a thickness of a pressuresensitive adhesive layer of 20 μm for protecting the circuit surface ofa semiconductor wafer.

“Young's modulus”, “Surface roughness”, “Stress relaxation ratio”, “DSC.peak amount” and “High temperature adhesion” of the above pressuresensitive adhesive sheet are shown in Table 1. In addition, thispressure sensitive adhesive sheet was used at the grinding of a siliconwafer, and “Wafer warpage” was measured. The results are shown in Table1.

The silicon wafer with the pressure sensitive adhesive sheet obtained inthe above described method was placed on a heating table with the topcoat layer of the pressure sensitive adhesive sheet facing the heatingtable, and a thermo-adhesive resin film (Adwill LP-3 made by LintecCorporation) was applied to the ground surface of the silicon wafer at180° C. by thermo-compression bonding. The pressure sensitive adhesivesheet did not adhere to the heating table even after cooled to roomtemperature, and no warpage occurred to the silicon wafer. Moreover, noproblem was observed when a gold (Au) film was formed by sputtering onthe ground surface of another silicon wafer which was ground separatelyin the similar manner.

Example 2

A pressure sensitive adhesive sheet was prepared in the same manner asin Example 1 except that BEAM SET 373A manufactured by Arakawa ChemicalIndustries Ltd. (a coating agent comprised of an acrylic polymer havingan ultraviolet ray-curable functional group, which contains 20% byweight of a silica filler in solids) was used for preparing the top coatlayer. The results are shown in Table 1.

In addition, no problem was observed either in the thermo-compressionbonding of a thermo-adhesive resin film or in the formation of an Aufilm by sputtering performed in the same manner as in Example 1.

Comparative Example 1

A pressure sensitive adhesive sheet was prepared in the same manner asin Example 1 except that a top coat layer was not formed in Example 1.

“Young's modulus”, “Surface roughness”, “Stress relaxation ratio” and“High temperature adhesion” of this pressure sensitive adhesive sheetare shown in Table 1. In addition, this pressure sensitive adhesivesheet was used at the grinding of a silicon wafer, and “Wafer warpage”was measured. The results are shown in Table 1.

In addition, the thermo-compression bonding of a thermo-adhesive resinfilm and the formation of an Au film by sputtering were performed in thesame manner as in Example 1. Although no warpage of the wafer occurredin both cases, the pressure sensitive adhesive sheet adhered to thetable and was hard to be released.

Comparative Example 2

A pressure sensitive adhesive sheet was prepared in the same manner asin Example 1 except that a coating agent for a top coat layer waschanged to a solution of 100 parts by weight of a non-crosslinkablepolystyrene thermoplastic elastomer (no filler added) in a solvent.

“Young's modulus”, “Surface roughness”, “Stress relaxation ratio”, “DSCpeak amount” and “High temperature adhesion” of the above pressuresensitive adhesive sheet are shown in Table 1. In addition, thispressure sensitive adhesive sheet was used at the grinding of siliconwafers, and “Wafer warpage” was measured. The results are shown in Table1.

In addition, the thermo-compression bonding of a thermo-adhesive resinfilm and the formation of an Au film by sputtering were performed in thesame manner as in Example 1. Although no warpage of the wafer occurredin both cases, the pressure sensitive adhesive sheet adhered to thetable and was hard to be released.

Comparative Example 3

A pressure sensitive adhesive sheet was prepared in the same manner asin Example 1 except that a base material was changed to a thermoplasticpolyethylene terephthalate film having a thickness of 188 μm, and a topcoat layer was not provided.

“Young's modulus”, “Surface roughness”, “Stress relaxation ratio” and“High temperature adhesion” of this pressure sensitive adhesive sheetare shown in Table 1. In addition, this pressure sensitive adhesivesheet was used at the grinding of a silicon wafer, and “Wafer warpage”was measured. The results are shown in Table 1.

In addition, the thermo-compression bonding of a thermo-adhesive resinfilm and the formation of an Au film by sputtering were performed in thesame manner as in Example 1. The warpage of a wafer was observed in theboth cases, and a part of the wafer broke.

Example 3

A pressure sensitive adhesive sheet for dicing a semiconductor wafer wasprepared in the same manner as in Example 1 except that the thickness ofa base material was changed to 80 μm. “Young's modulus”, “Surfaceroughness”, “Stress relaxation ratio”, “DSC peak amount” and “Hightemperature adhesion” of this pressure sensitive adhesive sheet areshown in Table 1.

In addition, the pressure sensitive adhesive sheet was used to fix awafer having a diameter of 200 mm which is ground to a thickness of 200μm to a wafer frame, and the wafer was diced to a size of 10 mm×10 mmusing a dicing apparatus. Subsequently, the wafer was heated by beingplaced on a hot plate of 180° C. for five minutes with the top coatlayer of the pressure sensitive adhesive sheet facing the hot plate. Thewafer was cooled for one hour at room temperature, taken out of the hotplate, and was subjected to the ultraviolet ray irradiation (250 mJ/cm )to the pressure sensitive adhesive layer from the top coat side of thepressure sensitive adhesive sheet. The pressure sensitive adhesive sheetwas expanded by 10 mm using an expanding apparatus to increase the chipspacing, and the chip was subjected to pickup by pushing up with aneedle from the pressure sensitive adhesive sheet side. All processeswere operated with no problem, without adhesion of the pressuresensitive adhesive sheet to the hot plate.

Example 4

A pressure sensitive adhesive sheet was prepared in the same manner asin Example 1 except that the coating agent for forming pressuresensitive layer was changed to an ultraviolet ray-curable pressuresensitive adhesive of the following formulation.

an adduct product of 100 parts by weight of a copolymer (weight averagemolecular weight of about 400,000) consisting of 60 parts by weight of2-ethylhexyl acrylate and 40 parts by weight of 2-hydroxyethyl acrylate;with 48 parts by weight of 2-methacryloyloxyethyl isocyanate

0.2 parts by weight of photopolymerization initiator comprised ofbis(2,4,6-trimethylbenzoyl)phenylphosphine oxide

0.3 parts by weight of a crosslinking agent comprised of an adductproduct of tolylenediisocyanate with trimethylolpropane

The results are shown in the following table 1.

Example 5

A pressure sensitive adhesive sheet was prepared in the same manner asin Example 2 except that the coating agent for forming pressuresensitive layer was changed to an ultraviolet ray-curable pressuresensitive adhesive of the following formulation.

100 parts by weight of a copolymer (weight average molecular weight ofabout 600,000) consisting of 90 parts by weight of n-butyl acrylate and10 parts by weight of acrylic acid

120 parts by weight of 6-functional urethane acrylate oligomer havingweight average molecular weight of 760

0.2 parts by weight of photopolymerization initiator comprised ofbis(2,4,6-trimethylbenzoyl)phenylphosphine oxide

15 parts by weight of a crosslinking agent comprised of an adductproduct of tolylenediisocyanate with trimethylolpropane

The results are shown in the following table 1.

Example 6

A pressure sensitive adhesive sheet was prepared in the same manner asin Example 2 except that the coating agent for forming pressuresensitive layer was changed to a removable pressure sensitive adhesiveof the following formulation.

84 parts by weight of n-butyl acrylate

10 parts by weigh of methylmethacrylate

1 part by weight of acrylic acid

5 parts by weight of 2-hydroxyethyl acrylate

15 parts by weight of a crosslinking agent comprised of an adductproduct of tolylenediisocyanate with trimethylolpropane

The results are shown in the following table 1. TABLE 1 Base materialTop coat layer Stress Young's Surface DSC peak relaxation High Wafermodulus roughness amount ratio temperature warpage (MPa) (μm) (J/g) (%)adhesion (mm) Example 1 200 0.11 <0.02 87 not adhered 5 Example 2 2000.15 <0.02 87 not adhered 5 Example 3 200 0.15 <0.02 87 not adhered —Comparative 200 0.06 — 87 adhered 5 Example 1 Comparative 200 0.12 0.3587 adhered 5 Example 2 Comparative 4900 0.04 — 30 not adhered 15 Example3 Example 4 200 0.11 <0.02 87 not adhered 5 Example 5 200 0.15 <0.02 87not adhered 5 Example 6 200 0.15 <0.02 87 not adhered 5

1. A pressure sensitive adhesive sheet comprising a base materialobtained by film-forming and curing a first curable resin, a top coatlayer formed on the base material by coating and curing a second curableresin, and a pressure sensitive adhesive layer formed on the oppositeside of the base material.
 2. The pressure sensitive adhesive sheetaccording to claim 1, wherein a cured resin forming the top coat layerhas no peak of 0.1 J/g or more in a DSC measurement from 50 to 200° C.3. The pressure sensitive adhesive sheet according to claim 1, whereinsaid base material has a Young's modulus of 50 to 5,000 MPa.
 4. A methodfor protecting the surface of a semiconductor wafer comprising the stepsof applying the pressure sensitive adhesive sheet of claim 1 to acircuit surface of a semiconductor wafer having circuitry formed on thefront side, and grinding a backside of the semiconductor wafer.
 5. Themethod for protecting the surface of a semiconductor wafer according toclaim 4, wherein a semiconductor wafer applied with a pressure sensitiveadhesive sheet is subjected to heat treatment or treatment involvingheat generation before or after grinding the semiconductor wafer.
 6. Themethod for protecting the surface of a semiconductor wafer according toclaim 5, wherein said heat treatment is thermo-compression bonding of athermo-adhesive film on the ground side of a semiconductor wafer.
 7. Themethod for protecting a semiconductor wafer according to claim 5,wherein the treatment involving heat generation is treatment selectedfrom vacuum deposition, sputtering and plasma etching applied to theground side of a semiconductor wafer.
 8. A method for processing aworkpiece comprising the steps of fixing a workpiece by the pressuresensitive adhesive sheet of claim 1 and picking up the workpiece.
 9. Themethod for processing a workpiece according to claim 8, wherein aworkpiece applied with a pressure sensitive adhesive sheet is subjectedto heat treatment or treatment involving heat generation, before thepicking up of the workpiece.
 10. The method for processing a workpieceaccording to claim 9, wherein a workpiece fixed to a pressure sensitiveadhesive sheet is applied with a thermo-adhesive film bythermo-compression bonding; the workpiece is diced together with thethermo-adhesive film; and then the diced workpiece is picked up andthermally adhered to a substrate via the thermo-adhesive film.
 11. Thepressure sensitive adhesive sheet according to claim 2, wherein saidbase material has a Young's modulus of 50 to 5,000 MPa.